Sheet-form abrasive articles, such as coated abrasives, are often used in combination with a back up pad. The back up pad provides support to the abrasive article during abrading. In general, a conventional back up pad used for this purpose typically comprises a pad facing comprising an attachment material such as knitted loop fabric, cloth, vinyl sheeting, hook fabric, and the secured to a resilient polyurethane open cell foam or "body member". The attachment material of the pad facing engages with the back side of the abrasive article to securely hold it during use. The resilient polyurethane foam or "body member" is used to lend certain desirable properties (strength, rigidity, life, flexibility and the like) to the abrasive article during its use, the need for such properties being dependent upon the intended abrading application.
The physical properties are dictated by the particular use application of the abrasive article. For instance, polyurethane foam backup pads in the abrasive article environment are subjected to high frequency cyclic compression during use. This produces heat within the foam. This heat generation, combined with the poor heat transfer of a foam, can produce a high internal temperature at the site of greatest deformation. Foam back up pads generally must be able to withstand internal temperatures of .gtoreq.90.degree. C. without permanent deformation and retain their shape without distorting or coming apart. Such distortion may induce thermal degradation of the foam with the formation of internal voids, followed by structural failure of the pad. Therefore, high elasticity of the foam pad is required to minimize heat generation due to hysteresis losses during cyclic compression of the foam during use. If excessive internal temperatures are developed during use, the foam can degrade and fail. The foam must also have high tensile strength and tear resistance to resist the stresses placed on the pad during high speed operation of the abrasive article. In addition, the backup pad must have the proper firmness to optimize abrasive performance, and the proper weight to be compatible with the driving tool (i.e., orbital sander counterweight). Also, foam back up pads need to display acceptable performance especially during random orbital usage. Such foam back up pads must have sufficient tensile strength and tear strength to resist failure at rotational speeds as high as 15,000 rpm. The ability of a foam pad to withstand such a rotational force provides an indirect measurement of the tensile strength and tear resistance of the foam. Generally, one important factor in determining the ultimate physical capabilities of polyurethane foam is its foam-forming chemistry.
The basic reaction chemistry for the formation of polyurethane or isocyanate-based resins involves a condensation reaction of isocyanate (NCO) and hydroxy (OH) end-groups. This forms a basic polymeric unit with urethane linkage groups (i.e., R.sub.a NHC(O)--OR.sub.b), from which the name to this class of materials was derived. Another reaction that plays an important role involves isocyanate (NCO) end-groups and water molecules (H.sub.2 O) which react to produce carbon dioxide (CO.sub.2), which serves as a blowing (foaming) agent for certain cellular products, and amine which further reacts to form disubstituted ureas linkages. In any event, the two major ingredients of polyurethane resin systems are liquid isocyanates as a source of NCO groups and polyols as a source of hydroxyl (OH) groups. Isocyanates used are generally difunctional (diisocyanates). Common examples include toluene (or tolylene) diisocyanate in two isomeric forms (2,4 and 2,6) which is abbreviated "TDI", and methylene di (or bis) phenyl diisocyanate which is abbreviated "MDI", also used in polymeric form ("PMDI"). Polyols, also referred to as "macroglycols", feature hydroxyl groups (OH) as end-group and side-group. The chain length of the polyol and frequency of occurrence of OH groups (functionality) can be varied. In general, flexible polyurethanes are associated with low functionality and long chains, while rigid ones correspond to high functionality and short chains. Polyols are generally divided into two classes: one being polyester types which generally have good resistance to oils and hydrocarbons, and polyether types, which generally have good resistance to hydrolysis (water-associated degradation). Polyurethane-type resins often feature other chemicals that play a role in the complex and varied chemical reactions associated with polyurethane chemistry. Briefly, these chemicals are often called extenders, chain-extending agents (e.g., short chain diols such as 1,4-butane diol), curative agents, cross-linking agents, or even catalysts as they are used in relatively small amounts. Conventional catalysts include, for example, amines such as tertiary amines, tin soaps and organic tin compounds. Nucleating agents, surfactants, and fire-retardants are also often added to foam forming formulations.
Commercially available abrasive article back up pads are known that include TDI/polyester foams and MDI/polyester foams. Traditionally, formulations based on polyester polyols and toluene diisocyanate (TDI) have been used in order to prepare foams for abrasive article back up pads as they were considered to provide a requisite combination of strength and elasticity. These formulations, however, suffer from several major disadvantages. One disadvantage being the high vapor pressure of TDI. As another disadvantage, polyester foam systems also incur high raw material costs, as polyester polyols are often significantly more expensive than other polyols such as polyether polyols. The addition of a prepolymer synthesis step makes this cost differential even greater, that is, a two-part formulation foam system used to form a polyurethane foam body member based on a polyol and toluene diisocyanate (TDI) quasi-prepolymer part and a polyester polyol/catalyst/water part. Another disadvantage is the high viscosity of polyester polyols. This requires the use of low pressure foam machines, which require frequent solvent flushes to remove urethane residues from the mechanical mixing components. Possibly more common are MDI/polyester foam pad formulations.
By way of explanation, conventional foam machines generally come in two varieties. The first type is a "low pressure" machine which relies on a mechanical mixing device in the dispensing head to mix the two component streams. The advantage of this type of machine is its ability to handle a wide range of viscosities and small volumes per foam shot. On the other hand, a major disadvantage is the need to regularly flush the mixing head with solvent to remove curing polyurethane residue from the mixing chamber. The solvent most commonly used for this is methylene chloride, which leads to hazardous waste material requiring costly handling and disposal. A second foam machine type is the "high pressure" or impingement mixing machine. In this type of device, mixing is achieved by impingement of two high velocity component streams within the mixing chamber. Because there is no mechanical mixing fixture to interfere, the chamber can be wiped clean with a piston after every shot, eliminating the need for solvent cleaning. This is currently the preferred type of machine for large, high volume parts such as automotive seating and RIM part manufacture. However, back up pads based on polyester/TDI foams are restricted to usage in low pressure foam machines due to the high viscosity of the polyester foam systems and volatility of TDI even though these machines require troublesome and time-consuming regular cleaning with noxious solvents. A need remains to find a less hazardous class of isocyanates for use in making foam back up pads.
MDI (methylene diphenyl diisocyanate)/polyester polyurethane foams are also known but they have serious drawbacks including solvent waste problems associated with cleaning low-pressure foam machines, as the more viscous polyester system requires resorting to the low pressure machines, and a foam elasticity that is lower than that generally considered acceptable for abrasive back up pad applications. These MDI/polyester foams are generally of lower performance than TDI/polyester foams, but are nonetheless widely utilized.
Other related art is set forth in the following references:
U.S. Pat. No. 4,631,220 (Clifton) pertains to a coated abrasive back up pad comprising a layer of resiliently compressible foam, a rigid fiber reinforced polymeric backing plate, a circular array of openings in the backing plate through which the backing plate is attached by screws to a drive mechanism, and a flexible adhesion layer fixed to an opposite surface of the foam layer. The back up pad includes a metal reinforced plate between the backing plate and the layer of foam.
U.S. Pat. No. 4,893,436 (Rich) describes a back up pad of felted polyurethane foam for use on a vibrating or oscillating sanding device that both efficiently transfers driving forces between the surfaces of the pad to drive a sheet of coated abrasive adhered to its outer surface against a work piece, and restricts chattering and bouncing of the sanding device on the work piece.
As apparent from the above, there remains a need for a back up pad foam affording favorable production safety and waste reduction while meeting backup pad performance requirements associated with the abrasive article environment.